1. Field of the Invention
The present invention relates to a method of crystallizing amorphous silicon. More particularly, the present invention relates to a method of crystallizing amorphous silicon using nanoparticles.
2. Description of the Related Art
Crystallization of amorphous silicon into polysilicon is essential to the formation of a high-performance thin film transistor (TFT) on a glass or plastic substrate. Use of this technology is essential in this regard because polysilicon has an electron mobility that is significantly higher than that of amorphous silicon and is thus used to form a driving circuit or a TFT on a glass substrate used as a substrate of a flat display, such as a liquid crystal display.
Among many conventional methods of crystallizing amorphous silicon into polysilicon, the most widely used method is an excimer laser annealing method, as illustrated in FIG. 1.
In the excimer laser annealing method, after amorphous silicon is deposited on a substrate, an excimer laser beam is radiated at the substrate to melt the amorphous silicon, thereby crystallizing the amorphous silicon. In this conventional method, the amount of molten amorphous silicon and a state of crystallization vary with the power of a laser beam, as shown in FIG. 1.
FIGS. 1(a) and 1(b), when the power of a laser beam is near threshold energy, only a small amount of amorphous silicon acts as a crystal seed. In FIG. 1(a), the power of a laser beam is appropriately adjusted to be nearly equal to a threshold energy density so that most of an amorphous silicon layer 13 on a substrate 11 is molten and only a small amount of unmolten amorphous silicon 15a acts as a crystal seed. In FIG. 1(b), the molten amorphous silicon layer 13 is gradually cooled around the unmolten amorphous silicon 15a and grows into ideal grains.
In FIGS. 1(c) and 1(d), at a power lower than the threshold energy, only a top surface of the amorphous silicon layer 13 is molten and is then cooled so that the amorphous silicon layer 13 grows into small grains. As shown in FIG. 1(c), when the power of a laser beam is lower than the threshold energy density, only the top surface of the amorphous silicon layer 13 is molten, and the amorphous silicon layer 13 near the substrate 11 remains without being molten 15b. Accordingly, when the amorphous silicon layer 13 is cooled, nonuniform grains are formed, as shown in FIG. 1(d).
In FIGS. 1(e) and 1(f), at a power higher than the threshold energy, the amorphous silicon layer 13 is completely molten so that no amorphous silicon remains to act as a crystal seed. Accordingly, seeds are generated nonuniformly and crystals grow nonuniformly. When the power of a laser beam is higher than the threshold energy density, the amorphous silicon layer 13 is completely molten as shown in FIG. 1(e). When it is cooled, crystals grow nonuniformly and the size of grains decreases, as shown in FIG. 1(f).
Consequently, in the conventional laser annealing technology, the power of a laser beam must be as near as possible to a threshold energy density so that a proper amount of amorphous silicon, which can act as a crystal seed, will remain. It is difficult, however, to adjust the energy density of a laser beam accurately and to control the number of crystal seeds and the size and arrangement thereof. Accordingly, grains are too small and grow nonuniformly.